1. Field of the Invention
The present invention relates to an adaptive antenna for a base station and a terminal unit used in for example a radio communication system.
2. Description of the Related Art
An adaptive antenna suppresses undesired signal such as delayed signals and interference signals that a base station or a terminal unit has received so as to increase the data transmission rate and the number of users. In the adaptive antenna, energy of delayed signals through multipath is combined as desired signals and thereby the signal-to-noise ratio of the desired signal is improved.
As shown in FIG. 9, signals received by a plurality of omni-directional antenna elements 101, 102, and 103 are sent to A/D converters 104, 105, and 106. The A/D converters 104, 105, and 106 convert the received signals into digital signals and distribute the digital signals to a plurality of adaptive signal processing portions 107, 108, and 109. In the adaptive signal processing portions 107, 108, and 109, the output signals of the A/D converters 104, 105, and 106 are sent to respective weighting units 110. The output signals of the weighting units 110 are sent to respective adding units 111. The adding units 111 combine the output signals of the weighting units 110.
A weighting amount of each weighting unit 110 is designated by a weight control circuit 113. The weight control circuit 113 designate weighting amounts of the weighting units 110 so as to emphasize a signal component that has a strong correlation with a reference signal and suppress the other signal components as interference components.
In addition, the weight control circuit 113 controls the weighting amounts that the adaptive signal processing portions 107, 108, and 109 designate in such a manner that a particular adaptive signal processing portion extracts a first incoming signal component (that does not have a delay) and other adaptive signal processing portions extract signal components that have delays.
Thus, a combining unit 112 extracts a pure signal of which delayed signals and interference signals are removed from a received signal that consist of a first incoming signal and delayed signals.
However, assuming that the number of delayed signals that the adaptive antenna receives is L and the number of antenna elements thereof is N, the adaptive antenna requires (L.times.N) weighting units. The number of weighting units affects the number of calculations of the weighting amounts of the controlling circuit. Thus, the circuit structure becomes complicated.